Might not be a good idea to install a Motion sensor in the CAR especially on the ceiling. Depending on how big the return grill is, vibration can set off a motion sensor due to resonance. Second, the sensor may need a clean field of view to cover the area being monitored. I take it this is for security reasons to turn on a video or an alarm?
There are alternate methods that can be used...such as an IR sensor that is available as a fake wall receptacle. The IR includes an X10 unit that triggers the remote monitor through a branch circuit as a power line carrier that connects to somewhere else in the house for a recording or alarm unit. You can use an RF transceiver also. These methods eliminate running LV through the house by locating the devices at standard 120V receptacles that are not on AFCI lines that need filtered coupling.
Most video surveillance systems now have image motion detection built right in the DVR so motion detection is a thing of the past for surveillance systems, but I believe the OP is talking about controlling lights, which the Leviton power pack relay can be mounted right in the fixture canopy, or switch location and a 18/3 conductor CL2 plenum rated LV cable routed to the fixture is all that is needed.
Leviton has a selection of options to fit most needs, and systems can be combined to control more lights, such as warehouses when you don't want only one light to come on at a time, most of the gas stations and or conievance store bathrooms I do, I used the 360
? Leviton ceiling system, It gives me allot of flexibility and there very good about not shutting off the lights when someone is in the bathroom, hubble also make a good system that is very simular to Leviton
